1. Field of the Invention
This invention relates generally to carriers and more particularly to devices adapted to carry large sheets of material, such as four foot by eight foot sheets of plywood or gypsum wallboard.
2. Description of the Prior Art
It is difficult for one person to single-handedly carry a sheet of plywood or gypsum wallboard because of the typically large dimensions of those materials. In recognition of this problem, various individuals have invented devices which make it substantially easier for a single person to carry large sheets of material.
One such invention is disclosed in U.S. Pat. No. 2,430,142 of S. C. Roberts. In his patent, Roberts teaches a device including an adjustable framework forming a lip at one end and forming a handle at the other end. When the lip of the device is hooked around an edge of a large sheet of material, the material can be lifted by exerting an upward force on the handle of the device.
In U.S. Pat. No. 2,651,441, R. S. Rau et al disclose a device including a substantially L-shaped member that is clipped onto a body harness. The L-shaped member engages the lower edge of a sheet of material so that the weight of the material is supported by the body harness.
M. I. Masterson, Jr., in U.S. Pat. No. 3,203,606, discloses a device for carrying wallboard including an elongated rod having a handle attached at a first end and a projecting plate attached to the other end. To use the device, a person grasps the handle in his hand, slips the plate beneath the lower edge of the wallboard and then lifts upwardly with the handle to pick up and then carry the wallboard.
Note that all of the aforementioned devices have a member that in some way engages the bottom edge of a sheet of material so that the material can then be lifted. The next two patents to be discussed, namely U.S. Pat. No. 3,524,670 of R. Ilich and U.S. Pat. No. 4,013,202 of H. Russo, operate on an entirely different principle, i.e., they both disclose devices that engage a sheet of material by compressively clamping members to the sides of the material.
Ilich's patent discloses a device that includes an inverted, J-shaped rod provided with a handle at its longer end and having a facing pair of gripping plates positioned within the curved portion proximate its other end. When an upward force is exerted on the handle, a linkage rod that couples the handle to the gripping plates forces the plates tightly together to firmly clamp the device to the sheet of material.
Russo's patent discloses a device including a base frame, a gripping frame pivotally attached to the base frame, and a pair of cams attached to the gripping frame. When a sheet of material is placed between the two frames and an upward force is exerted on the gripping frame, the cams tightly press the sheet of material against the base frame so as to clamp the device to the material and allow it to be lifted.
A problem not fully addressed by the prior art is how to design a device for carrying large sheets of material that will not allow the sheet of material to slip and possibly fall. For example, in the first three patents cited the members engaging the lower edge of a sheet of material could slip along or slip off that edge, thus allowing the sheet of material to fall to the ground. In the last two patents cited, should pressure be relaxed on either of the devices, even momentarily, the sheet of material would no longer be tightly clamped and it too could slip or fall.
A further problem that is not addressed by the prior art is how to design a carrying device that will not damage fragile sheets of material, such as plaster-filled wallboard (sometimes known as SHEETROCK.TM.). For instance, the wire framework of Robert's invention could cut the edges or the gripping members of Ilich's or Russo's inventions could dent the sides of a sheet of soft wallboard.